In the story, the protoganist doesn't send his girlfriend out to investigate the killer, he sends his male "housemate" of over twenty years. The way it was presented struck me as rather incongruous, so I did some research on Woolrich, and it turned out he was indeed gay, and there is no doubt in my mind now that while the presentation of the relationship between the characters is ambiguous, he intended his character to reflect this part of his personal identity as well.